Hybrid electric vehicles (HEVs) provide for increased fuel economy and reduced emissions compared to conventional vehicles with internal combustion engines, emerged as a very strong candidate to achieve these goals. A power-split hybrid system that uses planetary gear sets to connect an engine, a generator, and a motor, exhibits great potential to improve fuel economy by enabling the engine to operate at its most efficient operating region is shown in Prior Art FIG. 1.
Prior Art FIG. 1 illustrates a block diagram of a power-split hybrid electric vehicle (HEV) 20 and the power-split HEV's associated control system. The power-split hybrid system combines the benefits of both the parallel and series types of hybrid systems without sacrificing the cost effectiveness of the hybrid system. The power-split hybrid system has an internal combustion engine 22 connected to a planetary gear-set 24 having a carrier, a sun, and a ring gear, which can transmit torque to the wheels 26, 28 with the proper reaction torque of a generator 30 together with a traction motor 32. The traction motor 32 is used to supplement the wheel torque, similar to a parallel hybrid system. Since the generator 30 provides reaction torque to the engine 22, it can generate electricity for the traction motor 32, similar to a series hybrid system. A high voltage (HV) battery 34 acts as energy storage or additional power source device for the system 20. The power-split hybrid system also has the capability of driving the vehicle on electric power as well (a full hybrid electric vehicle). The two electric machines 30, 32 along with the engine 22 in this power-split hybrid architecture require a highly coordinated vehicle control system.
The power-split hybrid electric vehicle powertrain consists of two power sources: a combination of the engine 22, the generator 30 and the planetary gear set 24, and a combination of the motor 32 and the battery 34.
The planetary gear set 24 provides interconnection between the engine 22, the generator 30, and the motor 32, wherein the carrier gear is connected to the engine 22, sun gear is connected to the generator 30, and the ring gear is connected to the motor 32. The motor 32 is also connected to the wheels 26, 28 through gear reductions. This planetary gear configuration provides decoupling of the engine speed from the vehicle speed, which provides a great potential to achieve better engine efficiency.
The powertrain system consists of four subsystems/components including an engine subsystem, a transaxle subsystem, brake subsystem, and a battery subsystem. Each subsystem requires an associated controller to perform a respective specific function.
A transaxle subsystem controller module (TCM) 36 is integrated with the transaxle subsystem. The transaxle subsystem contains the planetary gear set 24 and the two electric machines, the motor 32 and the generator 30. The electric machines allow for both electrical and hybrid functionality and are used for different purposes depending on the driving conditions.
The brake subsystem 38 is an electro-hydraulic brake system, which provides the seamless integration of the friction brakes and regenerative braking functionality. To ensure that all these controllers work together to meet the driver's demand and provide desired energy management and functionality, a supervisory vehicle system controller is used.
The vehicle system controller (VSC) 40 communicates with each subsystem controller, and both manages and coordinates the drivetrain functions to satisfy a driver's request and to balance the energy flow to and from the multiple power units (engine, transaxle, and HV battery). The VSC 40 must balance the energy flow through the planetary gear-set 24 to provide various vehicle attributes. This is achieved by VSC 40 through various unique hybrid functionalities such as electric drive, regenerative braking, engine start-stop, hybrid drive, and HV battery power maintenance. For a given driver demand (through accelerator and brake pedal requests) and vehicle operation conditions, the VSC 40 maintains the vehicle at its most efficient operating point by managing the power among the various components of the vehicle 20 and coordinating the operating state of the engine 22, the generator 30, the motor 32, and the HV battery 34. In addition, the VSC 40 ensures the required vehicle's performance and drivability.
A sophisticated VSC 40 is required to achieve better fuel economy, emissions, and energy management without compromising vehicle's performance.
It is the responsibility of the VSC 40 to maintain the HV battery 34 at an optimum state of charge (SOC) by controlling the actual HV battery power. The actual HV battery power in this system is the result of the engine power, wheel power (or torque) and system losses. The optimum HV battery maintenance is achieved by constantly monitoring the HV battery SOC and calculating a desired HV battery power to achieve a target SOC. Once the VSC 40 has determined a desired HV battery power, it uses the desired HV battery power along with the driver power request (based on the driver inputs such as, accelerator and brake pedal) to calculate a feed-forward engine power. Feedback on the instantaneous HV battery power, using a conventional PI controller, controls the desired engine power such that the desired HV battery power is achieved. The desired engine power is finally split into desired engine torque and desired engine speed, which are then sent to the respective subsystem controllers.
FIG. 2 shows a power flow diagram of the power-split hybrid electric vehicle system 20. The vehicle 20 is capable of being driven in either an electric vehicle like drive mode (EV mode), or hybrid electric modes (HEV mode) such as, positive split, negative split, or parallel mode.
The power-split powertrain system provides a continuous variable transmission (CVT)-like functionality through the planetary gear set 24 and generator control to decouple the engine speed from the vehicle speed, and through the motor 32 that transmits part of the engine power from the engine electrical path (generator) to the wheels 26, 28. The CVT functionality achieves better engine efficiency and lower emissions by controlling the engine speed independent of the vehicle speed.
Use of nonlinear approaches have been used to control engine idle speed of non-hybrid vehicles. In a power-split hybrid vehicle, the engine speed is controlled independent of the vehicle speed to provide desired driver and HV battery power, which requires a sophisticated nonlinear vehicle system control algorithm. Due to the nonlinear behavior of the engine along with the engine response delay (which is a function of various environmental conditions) and engine inertial terms, the desired engine power is achieved differently under different driving conditions.
To control actual high-voltage (HV) battery power, a sophisticated controls system that controls engine power and thereby engine speed to achieve the desired HV battery maintenance power is provided. Conventional approaches use proportional-integral (PI) control systems to control the actual HV battery power in power-split HEV, which can sometimes result in either overshoots of engine speed and power or degraded response and settling times due to the nonlinearity of the power-split hybrid system.
Use of a conventional proportional integral (PI) controller, such as the PI-controller 44 shown in FIG. 3, for determining desired engine power may result in undesired engine speed response behavior under certain driving conditions. Such an undesired engine speed response is perceived by the driver as unintuitive response as it is caused by the conventional PI controller, and not by the driver's request.
Conventional control methods use linear control algorithms to control engine power, which can result in undesired engine speed behavior. The undesired behavior arises from the fact that a complete high fidelity mathematical model for the power-split HEV system along with the environmental effects cannot be accurately modeled inside a conventional controller.
Therefore, a modified controller adaptable to control nonlinear behaviors that does not require detailed knowledge of mathematical models of the engine power plant is needed to compensate for nonlinear behaviors associated with an engine power plant in a HEV.